As a wiring material for a semiconductor device, copper having a low resistance and a high electromigration resistance has been widely used instead of the conventional aluminum. In a multilayer wiring structure using copper as a wiring material, an upper layer wiring and a lower layer wiring are generally connected to each other by a via integrally formed in the upper layer wiring. Japanese Patent Laid-Open Publication No. 2004-207353 discloses such a connection structure.
In the above copper wiring, on the basis of the difference in thermal expansion coefficient between a copper film as a wiring and an interlayer insulating film in which a wiring is buried, the stress from the interlayer insulating film acts on the via. In addition, the stress from a wiring body also acts on the via. The larger the wiring width, the larger the stress acting on the via. The stress in the via is generated by the action of the stress from the interlayer insulating film. When the stress is generated in the via, copper atoms constituting the via are moved, whereby a void is generated in the bottom of the via. This phenomenon is widely known as stress migration, and contributes to the deterioration of the reliability of a wiring. Japanese Patent Laid-Open Publication No. 2004-296644 discloses the stress migration generated in a lower layer wiring.
FIG. 1 is a cross-sectional view showing a void generated in a via of a copper wiring.
An interlayer insulating film 102 is formed on a copper wiring 100 of a lower layer formed on a semiconductor substrate (not shown). A via hole 104 reaching the copper wiring 100 is formed in the interlayer insulating film 102.
The interlayer insulating film 102 has an upper layer copper wiring 106 formed thereon. The copper wiring 106 integrally has a via 108 buried in the via hole 104.
Copper atoms are moved by the stress in the via 108, whereby a void 110 is generated in the bottom of the via 108 as indicated by the dashed lines circle in the drawing.
Some copper wirings have a wide wiring part and a protruding wiring part, which has a smaller width than the wide wiring part and protrudes from the end of the wide wiring part along the extending direction of the wide wiring part.
FIG. 2 is a plan view showing a copper wiring having the above-mentioned protruding wiring part.
As illustrated, an upper layer copper wiring 112 has a wide wiring part 112a and a protruding wiring part 112b, which has a smaller width than the wide wiring part 112a and protrudes from the end of the wide wiring part 112a along the extending direction of the wide wiring-part 112a. 
A via 114 buried in an interlayer insulating film (not shown) under the copper wiring 112 is integrally formed at the end of the protruding wiring part 112b. The upper layer copper wiring 112 is connected to a lower layer copper wiring 116 through the via 114.
The “Stress migration phenomenon in narrow copper interconnects” (T. Suzuki et al., Journal of Applied Physics, Volume 101, U.S. Pat. No. 4,044,513, Feb. 15, 2007) discloses the stress generated in a via formed in a protruding wiring part.
However, in the related art, it has been difficult to reduce the failure, which is caused by stress migration and occurring in a via formed in a protruding wiring part. It has been found that in the protruding wiring part, the stress from the wide wiring part is concentrated on the via, whereby the stress migration easily occurs.